The structure type regmatch_t contains at least the following members:

regoff_trm_so
Byte offset from start of string to start of substring.

regoff_trm_eo
Byte offset from start of string of the first character after the end of substring.

regcomp()

The regcomp() function will compile the regular expression contained in the string pointed to by the pattern argument and place the results in the structure pointed to by preg. The cflags argument is the bitwise
inclusive OR of zero or more of the following flags, which are defined in the header <regex.h>:

REG_EXTENDED
Use Extended Regular Expressions.

REG_ICASE
Ignore case in match.

REG_NOSUB
Report only success/fail in regexec().

REG_NEWLINE
Change the handling of NEWLINE characters, as described in the text.

The default regular expression type for pattern is a Basic Regular Expression. The application can specify Extended Regular Expressions using the REG_EXTENDEDcflags flag.

If the REG_NOSUB flag was not set in cflags, then regcomp() will set re_nsub to the number of parenthesised subexpressions (delimited by \(\) in basic regular expressions or () in
extended regular expressions) found in pattern.

regexec()

The regexec() function compares the null-terminated string specified by string with the compiled regular expression preg initialized by a previous call to regcomp(). The eflags argument is
the bitwise inclusive OR of zero or more of the following flags, which are defined in the header <regex.h>:

REG_NOTBOL
The first character of the string pointed to by string is not the beginning of the line. Therefore, the circumflex character (^), when taken as
a special character, will not match the beginning of string.

REG_NOTEOL
The last character of the string pointed to by string is not the end of the line. Therefore, the dollar sign ($), when taken as a special character, will not match
the end of string.

If nmatch is zero or REG_NOSUB was set in the cflags argument to regcomp(), then regexec() will ignore the pmatch argument. Otherwise,
the pmatch argument must point to an array with at least nmatch elements, and regexec() will fill in the elements of that array with offsets of the substrings of string that correspond to the parenthesised subexpressions
of pattern: pmatch[i].rm_so will be the byte offset of the beginning and pmatch[i].rm_eo will be one greater
than the byte offset of the end of substring i. (Subexpression i begins at the ith matched open parenthesis, counting from 1.) Offsets in pmatch[0] identify the substring that corresponds to
the entire regular expression. Unused elements of pmatch up to pmatch[nmatch-1] will be filled with -1. If there are more than nmatch subexpressions
in pattern (pattern itself counts as a subexpression), then regexec() will still do the match, but will record only the first nmatch substrings.

When matching a basic or extended regular expression, any given parenthesised subexpression of pattern might participate in the match of several different substrings of string, or it might not match any substring even though the pattern as a whole did
match. The following rules are used to determine which substrings to report in pmatch when matching regular expressions:

1.
If subexpression i in a regular expression is not contained within another subexpression, and it participated in the match several times, then the byte offsets in pmatch[i] will delimit the last such match.

2.
If subexpression i is not contained within another subexpression, and it did not participate in an otherwise successful match, the byte offsets in pmatch[i] will be -1. A subexpression does not participate in the match when:

* or \{\} appears immediately after the subexpression in a basic regular expression, or *, ?, or {} appears immediately after the subexpression in an extended regular expression, and
the subexpression did not match (matched zero times)

or

| is used in an extended regular expression to select this subexpression or another, and the other subexpression matched.

3.
If subexpression i is contained within another subexpression j, and i is not contained within any other subexpression that is contained within j, and a match of subexpression j is reported in pmatch[j], then the match or non-match of subexpression i reported in pmatch[i] will be as described in 1. and 2. above,
but within the substring reported in pmatch[j] rather than the whole string.

4.
If subexpression i is contained in subexpression j, and the byte offsets in pmatch[j] are -1, then the pointers in pmatch[i] also will be -1.

5.
If subexpression i matched a zero-length string, then both byte offsets in pmatch[i] will be the byte offset of the character or NULL terminator
immediately following the zero-length string.

If, when regexec() is called, the locale is different from when the regular expression was compiled, the result is undefined.

If REG_NEWLINE is not set in cflags, then a NEWLINE character in pattern or string will be treated as an ordinary character. If REG_NEWLINE is set, then newline will be treated as an ordinary character except as follows:

1.
A NEWLINE character in string will not be matched by a period outside a bracket expression or by any form of a non-matching list.

2.
A circumflex (^) in pattern, when used to specify expression anchoring will match the zero-length string immediately after a newline in string, regardless of the setting of REG_NOTBOL.

3.
A dollar-sign ($) in pattern, when used to specify expression anchoring, will match the zero-length string immediately before a newline in string, regardless of the setting of REG_NOTEOL.

regfree()

The regfree() function frees any memory allocated by regcomp() associated with preg.

The following constants are defined as error return values:

REG_NOMATCH
The regexec() function failed to match.

REG_BADPAT
Invalid regular expression.

REG_ECOLLATE
Invalid collating element referenced.

REG_ECTYPE
Invalid character class type referenced.

REG_EESCAPE
Trailing \ in pattern.

REG_ESUBREG
Number in \digit invalid or in error.

REG_EBRACK[] imbalance.

REG_ENOSYS
The function is not supported.

REG_EPAREN\(\) or () imbalance.

REG_EBRACE
\{ \} imbalance.

REG_BADBR
Content of \{ \} invalid: not a number, number too large, more than two numbers, first larger than second.

REG_ERANGE
Invalid endpoint in range expression.

REG_ESPACE
Out of memory.

REG_BADRPT
?, * or + not preceded by valid regular expression.

regerror()

The regerror() function provides a mapping from error codes returned by regcomp() and regexec() to unspecified printable strings. It generates a string corresponding to the value of the errcode argument, which must be
the last non-zero value returned by regcomp() or regexec() with the given value of preg. If errcode is not such a value, an error message indicating that the error code is invalid is returned.

If preg is a NULL pointer, but errcode is a value returned by a previous call to regexec() or regcomp(), the regerror() still generates an error string corresponding
to the value of errcode.

If the errbuf_size argument is not zero, regerror() will place the generated string into the buffer of size errbuf_size bytes pointed to by errbuf. If the string (including the terminating NULL) cannot fit in the buffer, regerror() will truncate the string and null-terminate the result.

If errbuf_size is zero, regerror() ignores the errbuf argument, and returns the size of the buffer needed to hold the generated string.

If the preg argument to regexec() or regfree() is not a compiled regular expression returned by regcomp(), the result is undefined. A preg is no longer treated as a compiled regular expression
after it is given to regfree().

RETURN VALUES

On successful completion, the regcomp() function returns 0. Otherwise, it returns an integer value indicating an error as described in <regex.h>, and the content of preg is undefined.

On successful completion, the regexec() function returns 0. Otherwise it returns REG_NOMATCH to indicate no match, or REG_ENOSYS to indicate that the function is not supported.

Upon successful completion, the regerror() function returns the number of bytes needed to hold the entire generated string. Otherwise, it returns 0 to indicate that the function is not implemented.

The regfree() function returns no value.

ERRORS

No errors are defined.

USAGE

An application could use:

regerror(code,preg,(char *)NULL,(size_t)0)

to find out how big a buffer is needed for the generated string, malloc a buffer to hold the string, and then call regerror() again to get the string (see malloc(3C)). Alternately, it could allocate a fixed, static buffer that is big enough to hold most strings, and then use malloc() to allocate a larger buffer if it finds that this is too small.

EXAMPLES

Example 1: Example to match string against the extended regular expression in pattern.

The following demonstrates how the REG_NOTBOL flag could be used with regexec() to find all substrings in a line that match a pattern supplied by a user. (For simplicity of the example, very little error checking is done.)